The present invention relates to electronic integrated circuits and methods of fabrication, and, more particularly, to semiconductor integrated circuits on dice with transparent substrates.
Silicon-on-insulator substrates
Integrated circuits fabricated in silicon-on-insulator substrates offer performance advantages including freedom from latchup for CMOS structures, high packing density, low parasitic capacitance, low power consumption, radiation hardness, high voltage operation, and the possibility of three dimensional integration. Indeed, isolation trenches extending through the silicon layer down to the insulation provide a simple approach to dielectric isolation of integrated circuit devices. The sidewalls of such trenches are coated with an insulator, usually silicon dioxide ("oxide"), and the remaining portion of trench opening, if any, is filled with a filler which is usually polycrystalline silicon. Diffused PN junctions can also be used for lateral isolation.
Silicon-on-insulator wafer bounding as described by J. Lasky et al., Silicon-On-Insulator (SOI) by Bonding and Etch-Back, 1985 IEDM Tech. Deg. 684 (1985), proceeds as follows: a lightly doped epitaxial layer of silicon is grown on a heavily doped silicon substrate, oxide is thermally grown on the epilayer, a second lightly doped silicon substrate is thermally oxidized, the two oxidized surfaces are pressed together. See FIG. 1a. The pressed together wafers are inserted into an oxidizing atmosphere at 1,100.degree. C. to bond them as illustrated in FIG. 1b. Lastly, grinding and polishing or a preferential etch is used to remove the heavily doped substrate, leaving the thin, lightly doped epitaxially layer above the bonded thermally grown oxides which are now on the second substrate as shown in FIG. 1c. The resulting thin silicon layer above the thermally grown oxide has high quality and the oxide also retains its quality and may be thick, as might be desired for CMOS or high voltage devices, or thin, as might be desired for shared element applications. FIG. 1d heuristically illustrates trench isolation with poly filled trenches isolating MOSFET and bipolar devices.
Co-pending U.S. patent application Ser. No. 07/921,197, filed Jul. 28, 1992 (Linn, Lowry, Rouse, Buller, and Speece), discloses wafer bonding with the use of an oxidizer to lower the temperature of the oxidation bonding as illustrated in FIGS. 2a-c.
Direct wafer bonding of dissimilar materials is well known to those skilled in the art and not discussed in detail.
Integrated circuits formed in semiconductor substrates may incorporate various types of photodetectors. Incident light with sufficiently short wavelength generates electron-hole pairs in the semiconductor and thereby increases the conductivity of photoresistors or the current in photodiodes and phototransistors. The incident light typically enters through an opening in the wiring on the front side of the integrated circuit and forms electron-hole pairs near the surface (penetration of about 1 .mu.m for yellow-green light in silicon).
Integrated circuits also may be fabricated with light emission sources such as light emitting diodes or lasers, typically made of gallium arsenide type compound semiconductors. This permits optical communication between integrated circuits using optical fibers affixed to the light sources and detectors.
Temperature regulation of integrated circuits can compensate for the temperature coefficients of parameter change for circuits such as voltage references and current sources. However, known regulation methods of temperature sensing and heating a die containing an integrated circuit requires large amounts of power dissipation due to the vertical heat conduction out of a die. Various heat sensitive circuits such as bolometers have used circuits formed in suspended semiconductor films to provide thermal isolation; however, such suspension yields fragile structures.
Features
The present invention provides integrated circuits made in wafers of semiconductor layers bonded to glass substrates and with optically active devices coupling to backside light passing through the glass substrate. Glass substrates also provide enhanced thermal isolation for thermally regulated circuits.